Tell us about yourself!
I've done several of these and a 6 month lifetime seemed to be pretty consistent...whether used regularly or mostly sitting unused. I wonder why we had such different results. It could be the way we modified the Mont Blanc refill: I used a grinder and just removed a small amount of plastic from the end plug. If you cut it, instead, maybe it distorts the end plug and allows air inside?
jimdkc is correct that the Mont Blanc refill will dry out in a Pilot G2 body. However, my experience is that it takes about 6 months (rather than one month). My solution was to use a Pilot Metropolitan rollerball (rather than a G2). The Metropolitan is a "capped" pen. It's a bit less convenient to remove the cap before writing (rather than simply clicking), but the refill is not exposed to air except when you are writing and lasts several times longer.